Uplink pilot and signaling transmission in wireless communication systems

ABSTRACT

Techniques to more efficiently transmit pilot and signaling on the uplink in an OFDM system. With subband multiplexing, M usable subbands in the system are partitioned into Q disjoint groups of subbands. Each subband group may be assigned to a different terminal for uplink pilot transmission. Multiple terminals may transmit simultaneously on their assigned subbands. The transmit power for the pilot may be scaled higher to attain the same total pilot energy even though S instead of M subbands are used for pilot transmission by each terminal. Pilot transmissions from the terminals are received, and a channel estimate is derived for each terminal based on the pilot received on the assigned subbands. The channel estimate comprises a response for additional subbands not included in the assigned group. Subband multiplexing may also be used for uplink signaling transmission.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/340,507, entitled “Uplink Pilot And Signaling Transmission InWireless Communication Systems,” filed Jan. 10, 2003, which isincorporated herein by reference in its entirety for all purposes.

This application is related to U.S. patent application Ser. No.10/340,130, entitled “Channel Estimation for OFDM CommunicationSystems,” filed Jan. 10, 2003, which is incorporated herein by referencein its entirety for all purposes.

BACKGROUND

I. Field of the Invention

The present invention relates generally to data communication, and morespecifically to techniques for transmitting pilot and signaling (e.g.,rate control) information on the uplink in wireless communicationsystems.

II. Background

Wireless communication systems are widely deployed to provide varioustypes of communication such as voice, packet data, and so on. Thesesystems may be multiple-access systems capable of supportingcommunication with multiple users sequentially or simultaneously bysharing the available system resources. Examples of such multiple-accesssystems include code division multiple access (CDMA) systems, timedivision multiple access (TDMA) systems, and orthogonal frequencydivision multiple access (OFDMA) systems.

An OFDM system employs orthogonal frequency division multiplexing (OFDM)to effectively partition the overall system bandwidth into a number of(N) orthogonal subbands. These subbands are also referred to as tones,frequency bins, and frequency subchannels. Each subband may be viewed asan independent transmission channel that may be used to transmit data.

In a wireless communication system, an RF modulated signal from atransmitter may reach a receiver via a number of propagation paths. Thecharacteristics of the propagation paths typically vary over time due toa number of factors. For an OFDM system, the N subbands may experiencedifferent channel conditions and may achieve different signal-to-noiseratios (SNRs).

An accurate estimate of the response of the wireless channel between thetransmitter and the receiver is normally needed in order to effectivelytransmit data on the available subbands. Channel estimation is typicallyperformed by sending a pilot from the transmitter and measuring thepilot at the receiver. Since the pilot is made up of symbols that areknown a priori by the receiver, the channel response can be estimated asthe ratio of the received pilot symbols over the transmitted pilotsymbols.

Pilot transmission represents overhead in a wireless communicationsystem. Thus, it is desirable to minimize pilot transmission to theextent possible. However, because of noise and other artifacts in thewireless channel, a sufficient amount of pilot needs to be transmittedin order for the receiver to obtain a reasonably accurate estimate ofthe channel response. Moreover, because the contributions of thepropagation paths to the channel response and the propagation pathsthemselves typically vary over time, the pilot transmission needs to berepeated. The time duration over which the wireless channel may beassumed to be relatively constant is often referred to as a channelcoherence time. The repeated pilot transmissions need to be spacedsignificantly closer than the channel coherence time to maintain highsystem performance.

In the downlink of a wireless communication system, a single pilottransmission from an access point (or a base station) may be used by anumber of terminals to estimate the response of the distinct channelsfrom the access point to each of the terminals. In the uplink, thechannel from each of the terminals to the access point typically needsto be estimated through separate pilot transmissions from each of theterminals.

Thus, for a wireless communication system, multiple terminals may eachneed to transmit a pilot on the uplink to an access point. Moreover,signaling information such as rate control information andacknowledgments for downlink transmission may need to be sent on theuplink. If the uplink transmissions are performed in a time divisionmultiplexed (TDM) manner, then each terminal may be assigned a distincttime slot and would then transmit its pilot and signaling information inthe assigned time slot. Depending on the number of active terminals andthe duration of the time slots, a relatively large fraction of theuplink transmission time may be taken up by the pilot and signalingtransmissions. This inefficiency in the uplink transmission of pilot andsignaling information is exacerbated in an OFDM system where thedata-carrying capacity of the smallest transmission unit (typically oneOFDM symbol) may be quite large.

There is therefore a need in the art for techniques to transmit pilotand signaling information in a more efficient manner in wirelesscommunication systems (e.g., OFDM systems).

SUMMARY

Techniques are provided herein to more efficiently transmit pilot andsignaling on the uplink in wireless communication systems. With subbandmultiplexing, the M usable subbands in a system may be partitioned intoQ disjoint groups of subbands, where each subband is included in onlyone group, if at all. Each subband group may then be assigned to adifferent terminal. Multiple terminals may transmit simultaneously ontheir assigned subbands.

Using subband multiplexing, an accurate channel estimate may be obtainedfor each terminal for the entire usable band based on uplink pilottransmission on only a small subset of the usable subbands. If the totalenergy used for pilot transmission on S subbands is maintained so as toequal to the total energy otherwise used for pilot transmission on all Musable subbands, then it is possible to use the pilot transmission ononly S subbands to accurately interpolate the channel response for theother M−S subbands.

One embodiment provides a method for transmitting pilot on the uplink ina wireless communication system (e.g., an OFDM system) with a pluralityof subbands. In accordance with the method, M usable subbands suitablefor data transmission in the system are initially partitioned into Qdisjoint groups of subbands. The Q groups may include equal or differentnumber of subbands, and the subbands in each group may be uniformly ornon-uniformly distributed across the M usable subbands. A differentgroup of subbands is assigned to each of one or more terminals foruplink pilot transmission. Pilot transmission is then received from theone or more terminals on the assigned groups of subbands. For eachterminal, the transmit power for the pilot in each subband may be scaledhigher (e.g., by a factor of Q) so that the same total pilot energy isachieved even though the pilot transmission is over S instead of Msubbands. The power scaling may be performed such that the totaltransmit power available at each terminal is observed, transmit powerconstraints (e.g., regulatory constraints) are met, and hardwarecomponent costs are minimally increased (if at all). A channel estimatemay then be derived for each terminal based on the pilot received on thesubbands assigned to the terminal. The channel estimate for eachterminal can cover one or more additional subbands not included in thegroup assigned to the terminal. For example, the channel estimate mayinclude the response for all M usable subbands.

Subband multiplexing may also be used for transmission of signalinginformation on the uplink. The signaling information may comprise ratecontrol information used for downlink data transmission, acknowledgmentfor data received on the downlink, and so on.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 illustrates an OFDM system that supports a number of users;

FIGS. 2, 3, and 4 illustrate a frame structure, an OFDM subbandstructure, and an OFDM subband structure that supports subbandmultiplexing, respectively;

FIG. 5 shows a process for transmitting uplink pilot using subbandmultiplexing;

FIG. 6 illustrates a frame structure that supports subband multiplexingfor uplink pilot and signaling transmission;

FIG. 7 is a block diagram of an access point and a terminal in the OFDMsystem; and

FIGS. 8A through 8C show plots of potential savings that may be realizedwith subband multiplexing for uplink pilot and signaling transmission.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” An embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

The techniques described herein to transmit pilot and signalinginformation may be used in various types of wireless communicationsystem. For example, these techniques may be used for CDMA, TDMA, FDMA,and ODFM systems. These techniques may also be used for hybrid systemssuch as an OFDM TDM system that transmits pilot/signaling and trafficdata using time division multiplexing, whereby OFDM is used forpilot/signaling and another transmission scheme is used for trafficdata. For clarity, these techniques are specifically described below foran OFDM system.

FIG. 1 illustrates an OFDM system 100 that supports a number of users.OFDM system 100 includes a number of access points (AP) 110 that supportcommunication for a number of terminals (T) 120. For simplicity, onlyone access point is shown in FIG. 1. An access point may also bereferred to as a base station or some other terminology.

Terminals 120 may be dispersed throughout the system. A terminal mayalso be referred to as a mobile station, a remote station, an accessterminal, a user equipment (UE), a wireless device, or some otherterminology. Each terminal may be a fixed or a mobile terminal that cancommunicate with one or possibly multiple access points on the downlinkand/or uplink at any given moment. The downlink (or forward link) refersto transmission from the access point to the terminal, and the uplink(or reverse link) refers to transmission from the terminal to the accesspoint.

In FIG. 1, access point 110 communicates with user terminals 120 athrough 120 f via the downlink and uplink. Depending on the specificdesign of the OFDM system, an access point may communicate with multipleterminals simultaneously (e.g., via multiple subbands) or sequentially(e.g., via multiple time slots).

FIG. 2 illustrates a frame structure 200 that may be used for the OFDMsystem if a single frequency band is used for both the downlink anduplink. In this case, the downlink and uplink can share the samefrequency band using time division duplexing (TDD).

As shown in FIG. 2, downlink and uplink transmissions occur in units of“MAC frames”. Each MAC frame may be defined to cover a particular timeduration. Each MAC frame is partitioned into a downlink phase 210 and anuplink phase 220. Downlink transmissions to multiple terminals may bemultiplexed using time division multiplex (TDM) on the downlink phase.Similarly, uplink transmissions from multiple terminals may bemultiplexed using TDM on the uplink phase. For the specific TDMimplementation shown in FIG. 2, each phase is further partitioned into anumber of time slots (or simply, slots) 230. The slots may have fixed orvariable durations, and the slot duration may be the same or differentfor the downlink and uplink phases. For this specific TDMimplementation, each slot 230 in the uplink phase includes a pilotsegment 232, a signaling segment 234, and a data segment 236. Segment232 is used to send an uplink pilot from the terminal to the accesspoint, segment 234 is used to send signaling (e.g., rate control,acknowledgment, and so on), and segment 236 is used to send data.

The slots in the uplink phase of each MAC frame may be assigned to oneor more terminals for uplink transmission. Each terminal would thentransmit on its assigned slot(s).

Frame structure 200 represents a specific implementation that may beused for the OFDM system if only one frequency band is available. If twofrequency bands are available, then the downlink and uplink may betransmitted on separate frequency bands using frequency division duplex(FDD). In this case, the downlink phase may be implemented on onefrequency band, and the uplink phase may be implemented on the otherfrequency band.

The pilot and signaling transmission techniques described herein may beused for both TDD-based and FDD-based frame structures. For simplicity,these techniques are described specifically for the TDD-based framestructure.

FIG. 3 illustrates an OFDM subband structure 300 that may be used forthe OFDM system. The OFDM system has an overall system bandwidth of WMHz, which is partitioned into N orthogonal subbands using OFDM. Eachsubband has a bandwidth of W/N MHz. Of the N total subbands, only Msubbands are used for data transmission, where M<N. The remaining N−Msubbands are not used and serve as guard bands to allow the OFDM systemto meet its spectral mask requirements. The M “usable” subbands includesubbands F through M+F−1.

For OFDM, the data to be transmitted on each subband is first modulated(i.e., symbol mapped) using a particular modulation scheme selected foruse for that subband. For the N−M unused subbands, the signal value isset to zero. For each symbol period, the M modulation symbols and N−Mzeros for all N subbands are transformed to the time domain with aninverse fast Fourier transform (IFFT) to obtain a transformed symbolthat comprises N time-domain samples. The duration of each transformedsymbol is inversely related to the bandwidth of each subband. Forexample, if the system bandwidth is W=20 MHz and N=256, then thebandwidth of each subband is 78.125 KHz and the duration of eachtransformed symbol is 12.8 μsec.

OFDM can provide certain advantages, such as the ability to combatfrequency selective fading, which is characterized by different channelgains at different frequencies of the overall system bandwidth. It iswell known that frequency selective fading causes inter-symbolinterference (ISI), which is a phenomenon whereby each symbol in areceived signal acts as distortion to subsequent symbols in the receivedsignal. The ISI distortion degrades performance by impacting the abilityto correctly detect the received symbols. Frequency selective fading canbe conveniently combated with OFDM by repeating a portion of (orappending a cyclic prefix to) each transformed symbol to form acorresponding OFDM symbol, which is then transmitted.

The length of the cyclic prefix (i.e., the amount to repeat) for eachOFDM symbol is dependent on the delay spread of the wireless channel.The delay spread for a given transmitter is the difference between theearliest and latest arriving signal instances at a receiver for thesignal transmitted by this transmitter. The delay spread for the systemis the expected worst case delay spread for all terminals in the system.To effectively combat ISI, the cyclic prefix should be longer than thedelay spread.

Each transformed symbol has a duration of N sample periods, where eachsample period has a duration of (1/W) μsec. The cyclic prefix may bedefined to comprise Cp samples, where Cp is an integer selected based onthe expected delay spread of the system. In particular, Cp is selectedto be greater than or equal to the number of taps (L) for the impulseresponse of the wireless channel (i.e., Cp≧L). In this case, each OFDMsymbol would include N+Cp samples, and each symbol period would spanN+Cp sample periods.

In some OFDM systems, pilots are transmitted on the uplink by theterminals to allow the access point to estimate the uplink channel. Ifthe TDD-TDM frame structure shown in FIG. 2 is used, then each terminalcan transmit its uplink pilot in the pilot segment of its assigned slot.Typically, each terminal transmits the uplink pilot in all M usablesubbands and at full transmit power. This would then allow the accesspoint to estimate the uplink channel response across the entire usableband. Although this uplink pilot transmission scheme is effective, it isalso inefficient since a relatively large fraction of the uplink phasemay be used for pilot transmissions by all active terminals. The pilotsegments for all active terminals may comprise a large fraction of theuplink phase.

Techniques are provided herein to more efficiently transmit pilot on theuplink in the OFDM system. To be effective, a pilot transmission schemeneeds to be designed such that accurate channel estimates can beobtained for each active terminal based on the uplink pilot transmissionfrom the terminal. However, it has been discovered that the quality ofthe channel estimates is generally determined by the total energy of thepilot rather than the specifics of the pilot transmission scheme. Thetotal pilot energy is equal to the transmit power used for the pilotmultiplied by the time duration of the pilot transmission.

An accurate channel estimate may be obtained for the entire usable bandbased on pilot transmission on only S subbands, where S is selected suchthat Cp≦S<M and is typically much less than M. One such channelestimation technique is described in the aforementioned U.S. ProvisionalPatent Application Ser. No. 60/422,638, U.S. Provisional PatentApplication Ser. No. 60/422,362, and U.S. patent application Ser. No.[Attorney Docket No. 020718]. In fact, it can be shown that if the totalenergy used for pilot transmission on the S subbands is equal to thetotal energy used for pilot transmission on all M subbands, then it ispossible to accurately interpolate the channel response for the otherM−S subbands based on the pilot transmission on the S subbands using thechannel estimation technique above. In other words, if the total pilotenergy is the same, then the interpolated channel response for the M−Ssubbands would typically have the same quality (e.g., the same averagemean squared error) as the channel estimate obtained based on pilottransmission on all M subbands.

Subband multiplexing may be used to allow multiple terminals to transmitpilot simultaneously on the uplink. To implement subband multiplexing,the M usable subbands may be partitioned into Q disjoint groups ofsubbands such that each usable subband appears in only one group, if atall. The Q groups may include the same or different numbers of subbands,and the subbands in each group may be uniformly or non-uniformlydistributed across the M usable subbands. It is also not necessary touse all M subbands in the Q groups (i.e., some usable subbands may beomitted from use for pilot transmission).

In an embodiment, each group includes S subbands, where S=└M/Q┘ andS≧Cp, where “└ ┘” denotes the floor operator. The number of subbands ineach group should be equal to or greater than the delay spread Cp sothat the effects of ISI can be mitigated and a more accurate channelestimate can be obtain.

FIG. 4 illustrates an embodiment of an OFDM pilot structure 400 that maybe used for the OFDM system and which supports subband multiplexing. Inthis embodiment, the M usable subbands are initially divided into Sdisjoint sets, with each set including Q consecutive subbands. The Qsubbands in each set are assigned to the Q groups such that the i-thsubband in each set is assigned to the i-th group. The S subbands ineach group would then be uniformly distributed across the M usablesubbands such that consecutive subbands in the group are separated by Qsubbands. The M subbands may also be distributed to the Q groups in someother manners, and this is within the scope of the invention.

The Q groups of subbands may be assigned to up to Q terminals for uplinkpilot transmission. Each terminal would then transmit the pilot on itsassigned subbands. With subband multiplexing, up to Q terminals maysimultaneously transmit pilots on the uplink on up to M usable subbands.This can greatly reduce the amount of time needed for uplink pilottransmission.

To allow the access point to obtain high quality channel estimates, eachterminal may increase the transmit power per subband by a factor of Q.This would result in the total pilot energy for the pilot transmissionon the S assigned subbands to be the same as if all M subbands were usedfor pilot transmission. The same total pilot energy would allow theaccess point to estimate the channel response of the entire usable bandbased on a subset of the M usable subbands with little or no loss inquality, as described below.

The OFDM system may be operated in a frequency band that has a per MHzpower constraint of P dBm/MHz and a total power constraint of P·W dBm.For example, the 5 GHz UNII band includes three 20 MHz frequency bandsdesignated as UNII-1, UNII-2, and UNII-3. These three frequency bandshave total transmit power restrictions of 17, 24, and 30 dBm and per MHzpower restrictions of 4, 11 and 17 dBm/MHz, respectively. The powerconstraints per terminal may be selected based on the lowest powerconstraints for the three frequency bands, so that the per MHz powerconstraint is P=4 dBm/MHz and the total power constraint is P·W=17 dBm.

The groups of subbands may be formed such that full transmit power maybe used for uplink pilot transmission even if the per MHz and totalpower constraints are imposed on each terminal. In particular, if thespacing between the subbands within each group is approximately 1 MHz,then each terminal can transmit the uplink pilot on all S subbandsassigned to it at a power per subband of P dBm, and still abide by theper MHz power constraint. The total transmit power for the S subbandswould then be equal to P·S dBm, which is approximately equal to P·W dBmsince S≈W due to the 1 MHz spacing. In general, the per MHz and totalpower constraints can be met by appropriate scaling as long as S>W,where W is given in units of MHz.

In an exemplary OFDM system, the system bandwidth is W=20 MHz, N=256,and M=224. The OFDM pilot structure includes Q=12 groups, with eachgroup including S=18 subbands. For this pilot structure, 216 of the 224usable subbands may be used simultaneously for uplink pilot transmissionand the remaining 8 subbands are not used.

In general, the amount of transmit power that may be used for eachsubband in each group is dependent on various factors such as (1) theper MHz and total power constraints and (2) the distribution of thesubbands in each group. The terminals may transmit the uplink pilot atfull power even if the spacing between the subbands is not uniformand/or is less than 1 MHz. The specific amounts of power to use for thesubbands would then be determined based on the distribution of thesubbands among the Q groups. For simplicity, the S subbands in eachgroup are assumed to be uniformly spaced and separated by the requiredminimum spacing (e.g., at least 1 MHz).

FIG. 5 is a flow diagram of an embodiment of a process 500 fortransmitting uplink pilot using subband multiplexing. Initially, the Musable subbands are partitioned into Q disjoint groups of subbands (step512). This partitioning may be performed once based on the expectedloading in the OFDM system. Alternatively, the M usable subbands may bedynamically partitioned whenever warranted by changes in the systemloading. For example, fewer groups may be formed under light systemloading and more groups may be formed during peak system loading. In anycase, the partitioning is such that the condition S≧Cp is satisfied foreach group.

One group of subbands is assigned to each active terminal for uplinkpilot transmission (step 514). The subband assignment may be determinedat call setup or at a later time, and may be signaled to the terminal.Thereafter, each terminal transmits pilot on the uplink on its assignedsubbands (step 522). Each terminal may also scale up the transmit powerused for uplink pilot transmission, with the amount of transmit powerused for each subband being determined based on the various factorsnoted above. The amount of transmit power to use for each subband (oreach group of subband) may also be specified by the access point andsignaled to the terminal along with the subband assignment.

The access point receives uplink pilot transmissions from all activeterminals on all or a subset of the M usable subbands (step 532). Theaccess point then processes the received signal to obtain per-subbandchannel estimate for the subbands assigned to each active terminal (step534). For each active terminal, the channel estimate for the entireusable band may then be derived based on the per-subband channelestimate obtained for the assigned subbands (step 536). The channelestimate for the entire usable band may be derived from the channelestimate for a subset of the usable subbands using various techniques.One such channel estimation technique is described in the aforementionedU.S. Provisional Patent Application Ser. No. 60/422,638, U.S.Provisional Patent Application Ser. No. 60/422,362, and U.S. patentapplication Ser. No. [Attorney Docket No. 020718]. The channel estimatefor the entire usable band may also be derived by interpolating theper-subband channel estimate for a subset of the usable subbands.

For each active terminal, the channel estimate for the entire usableband may thereafter be used for downlink and/or uplink data transmissionto/from the terminal (step 538). The uplink pilot transmission andchannel estimation are typically continually performed during acommunication session to obtain up-to-date channel estimates.

The model for an OFDM system may be expressed as:r=H∘x+n,   Eq (1)where

-   -   r is a vector with N entries for the symbols received on the N        subbands;    -   x is a vector with N entries for the symbols transmitted on the        N subbands (some entries may include zeros);    -   H is an (N×1) vector for the channel frequency response between        the access point and terminal;    -   n is an additive white Gaussian noise (AWGN) vector for the N        subbands; and    -   “∘” denotes the Hadmard product (i.e., point-wise product, where        the i-th element of r is the product of the i-th elements of x        and H ).        The noise n is assumed to have zero mean and a variance of σ².

With subband multiplexing, each active terminal transmits pilot on its Sassigned subbands during the pilot transmission interval. Thetransmitted pilot for each terminal may be denoted by an (N×1) vectorx_(i), which includes a pilot symbol for each of the S assigned subbandsand zeros for all other subbands. The transmit power for the pilotsymbol for each assigned subband may be expressed as P_(UL)=x_(i,j) ²,where x_(i,j) is the pilot symbol transmitted on the j-th subband byterminal i.

A per-subband channel estimate Ĥ_(i) ^(meas) for terminal i may beexpressed as:Ĥ _(i) ^(meas) =r _(i) /x _(i) =H _(i) +n _(i) /x _(i),   Eq (2)where Ĥ_(i) ^(meas) is an (S×1) vector and a_(i)/b_(i)=[a₁/b₁ . . .a_(S)/b_(S)]^(T), which includes ratios for the S subbands assigned toterminal i. The per-subband channel estimate Ĥ_(i) ^(meas) may bedetermined by the access point for terminal i based on the received andtransmitted pilot symbols for each of the S subbands assigned to theterminal. The per-subband channel estimate Ĥ_(i) ^(meas) is thusindicative of the channel frequency response for terminal i for the Sassigned subbands.

An estimate for H in equation (1) may be obtained from the per-subbandchannel estimate Ĥ_(i) ^(meas) using several techniques. One suchtechnique, as noted above, is described in the aforementioned U.S.Provisional Patent Application Ser. No. 60/422,638, U.S. ProvisionalPatent Application Ser. No. 60/422,362, and U.S. patent application Ser.No. [Attorney Docket No. 020718].

If all N subbands are used for data transmission (i.e., M=N), it can beshown that the mean square error (MSE) for the channel estimate obtainedbased on pilot transmission on only S subbands using the techniquedescribed in the aforementioned U.S. Provisional Patent Application Ser.No. 60/422,638, U.S. Provisional Patent Application Ser. No. 60/422,362,and U.S. patent application Ser. No. [Attorney Docket No. 020718] is thesame as the MSE for the channel estimate obtained based on pilottransmission on all N subbands, if the following conditions aresatisfied:

-   -   1. Choose S≧Cp and S≧W;    -   2. Uniform distribution of the S subbands in each group across        the N total subbands; and    -   3. Set the transmit power for each of the S assigned N/S times        higher than the average transmit power P_(avg) defined below.

The total transmit power that may be used for transmission by a terminalis normally constrained by the lesser of (1) the total transmit powerP_(total) of the terminal (which may be limited by the terminal's poweramplifier) and (2) the total power constraint P·W of the operating band.The average transmit power P_(avg) is then equal to the smaller ofP_(total)/N and P·W/N. For example, P_(avg)=P·W/N if the total transmitpower that may be used by the terminal is limited by regulatoryconstraints

If only a subset of the N total subbands is used for data transmission(i.e., M<N), which is the case if some subbands are used for guardbands, then the minimum mean square error (MMSE) is only attained ifS=M. However, it has been found in the aforementioned U.S. ProvisionalPatent Application Ser. No. 60/422,638, U.S. Provisional PatentApplication Ser. No. 60/422,362, and U.S. patent application Ser. No.[Attorney Docket No. 020718] that if S≈1.1 Cp then the MSE is close tothe MMSE. Hence, for the case in which S≦M<N, the MSE is minimized forthe channel estimate obtained based on pilot transmission on only Ssubbands, if the following conditions are satisfied:

-   -   1. Choose S≈1.1 Cp and S>W;    -   2. Uniformly distribute the S subbands in each group across the        M data subbands; and    -   3. Set the transmit power for each of the S assigned subbands        N/S times higher than the average transmit power P_(avg)        described above.

In many wireless systems, the terminals may need to send signalinginformation on the uplink to the access point. For example, theterminals may need to inform the access point of the rate(s) to use fordownlink data transmission, send acknowledgment for received datapackets, and so on. The signaling information typically comprises asmall amount of data, but may need to be sent in a timely manner, andpossibly on a regular basis.

In some systems, rate control information may need to be sent on theuplink to indicate the rate that may be used on the downlink for each ofone or more transmission channels. Each transmission channel maycorrespond to a spatial subchannel (i.e., an eigenmode) in amultiple-input multiple-output (MIMO) system, a subband or frequencysubchannel in an OFDM system, a time slot in a TDD system, and so on.Each terminal may estimate the downlink channel and determine themaximum rate that may be supported by each of the transmission channels.Rate control information for the transmission channels may then be sentback to the access point and used to determine the rate for downlinkdata transmission to the terminal. The rate control information may bein the form of one or more rate codes, each of which may be mapped to aspecific combination of code rate, modulation scheme, and so on.Alternatively, the rate control information may be provided in someother form (e.g., the received SNR for each transmission channel). Inany case, the rate control information for each transmission channel maycomprise 3 to 4 bits, and the rate control information for alltransmission channels may comprise a total of 15 bits.

As another example, channel response or frequency selectivityinformation may need to be reported back to the access point. The numberof bits required for the channel response or frequency selectivityinformation may be dependent on the granularity of the information beingsent (e.g., every subband, or every n-th subband).

Techniques are also provided herein to more efficiently transmitsignaling information on the uplink in the OFDM system. The M usablesubbands may be partitioned into a number of Q_(R) disjoint groups,where each usable subband appears in only one group, if at all. TheQ_(R) groups may include the same or different number of subbands. Thegrouping of the usable subbands for uplink signaling information may bethe same or different from the grouping of the usable subbands foruplink pilot transmission. Each subband group may be allocated to oneterminal for uplink signaling transmission. Multiple terminals maytransmit signaling information simultaneously on their assignedsubbands.

The use of subband multiplexing to send uplink signaling information mayprovide various benefits. Because of the relatively large data-carryingcapacity of an OFDM symbol, it may be extremely inefficient to allocateentire OFDM symbols to active terminals when only a small amount of dataneeds to be sent. Using subband multiplexing, the number of subbandsallocated to each active terminal may be commensurate with the amount ofdata that needs to be sent.

The savings provided by subband multiplexing may be even greater if thetransmit power per subband is increased by the number of terminalsmultiplexed together within the same time interval. The higher transmitpower per subband would result in higher received SNR at the accesspoint, which would then support a higher order modulation scheme. Thiswould in turn allow more data or information bits to be transmitted oneach subband. Alternatively, each terminal may be assigned fewersubbands so that more terminals may be multiplexed together in the sametime interval. The fewer subbands can provide the requisitedata-carrying capacity if a higher order modulation scheme is used.

Subband multiplexing may also be used for the transmission ofacknowledgment on the uplink. For some systems, an acknowledgment mayneed to be sent by the receiver to acknowledge correct or erroneousdetection of each packet received by the receiver. Improved systemefficiency may be achieved by reducing the granularity of the allocationof resources for acknowledgment transmission (i.e., by assigning a groupof subbands instead of entire OFDM symbol to each terminal).

The amount of data to send for acknowledgment may differ from terminalto terminal and also from frame to frame. This is because each terminaltypically only sends acknowledgments for packets received in thecurrent/prior MAC frame, and the number of packets sent to each terminalcan differ among terminals and over time. In contrast, the amount ofdata to send for rate control tends to be more constant.

A number schemes may be used to allocate subbands for uplinktransmission of variable amounts of signaling (e.g., acknowledgment)among active terminals. In one scheme, the M usable subbands arepartitioned into a number of Q_(A) disjoint groups. The Q_(A) groups mayinclude the same or different number of subbands. Each active terminalmay be assigned a variable number of subbands for acknowledgmenttransmission. For this scheme, the number of subbands assigned to agiven terminal may be proportional to the number of packets sent to theterminal.

In another scheme, each active terminal is assigned a fixed number ofsubbands for acknowledgment transmission. However, the modulation schemeused by each terminal is not fixed, but can be selected based on thechannel conditions. For a reciprocal channel whereby the downlink anduplink are highly correlated, the transmission capacities of thedownlink and uplink are related. Thus, if more packets can be sent onthe downlink within a given time period because of improved channelconditions, then the same channel conditions can support thetransmission of more information bits on the uplink in a given timeinterval. Thus, by allocating a fixed number of subbands to each activeterminal but allowing the modulation to adapt based on the channelconditions, more acknowledgment bits may be sent when needed.

To simplify the assignment of subbands to active terminals, the subbandsmay be arranged into groups, and the terminals may be assigned groups ofsubbands instead of individual subbands. In general, each group mayinclude any number of subbands, depending on the desired granularity forthe subband assignment. As an example, 37 groups of subbands may beformed, with each group including 6 subbands. A given terminal may thenbe assigned any number of subband groups, depending on its datarequirement.

For a specific OFDM system design, between 150 and 2000 bits may betransmitted in two OFDM symbols for a range of rates supported by thesystem. This range of bit rates is also achieved under the assumptionthat higher transmit power is used for each subband with subbandmultiplexing. Each of the 37 subband groups for the example describedabove may then be used to send 150/37 to 2000/37 bits foracknowledgments, depending on the channel conditions. Thus, the fixednumber of subbands in each group may be able to send a variable numberof bits for acknowledgment, depending on the rate selected for use,which in turn depends on the channel conditions.

There may be instances where the transmit power per subband needs to bemaintained at the same level as for data transmission. This situationmay arise, for example, if all of the usable subbands are allocated to asingle terminal. However when the subbands have lower data-carryingcapacity, the requirement on it is also correspondingly lower. Two OFDMsymbols may be adequate for acknowledgment data for all expected channelconfigurations.

In an alternative scheme, acknowledgment data is sent along with uplinkpacket data. Additional delay may be incurred for the acknowledgmentdata if it needs to wait for packet data to be sent on the uplink. Ifthe additional delay is tolerable, then the acknowledgment data may besent with essentially no overhead since the amount of acknowledgmentdata is typically small and will likely fit in the padding portion of anuplink data packet.

In yet another scheme, acknowledgment data is sent along with the ratecontrol information. The group of subbands assigned to each activeterminal for rate control transmission may have greater data-carryingcapacity than that needed to send the rate control information. In thiscase, the acknowledgment data may be sent in the excess data-carryingcapacity of the subbands allocated for rate control.

When subband multiplexing is used for transmission of signalinginformation on the uplink, the access point can process the receivedsignal to individually recover the signaling (e.g., rate control andacknowledgment) sent by each terminal.

FIG. 6 illustrates an embodiment of a frame structure 600 that supportssubband multiplexing for uplink pilot and signaling transmission. TheMAC frame is partitioned into a downlink phase 610 and an uplink phase620. The uplink phase is further partitioned into a pilot segment 622, asignaling segment 624, and a number of slots 630. Subband multiplexingmay be used for segment 622 so that multiple terminals can concurrentlytransmit pilot on the uplink in this segment. Similarly, subbandmultiplexing may be used for segment 624 so that multiple terminals canconcurrently transmit signaling (e.g., rate control information,acknowledgment, and so on) on the uplink in this segment. Slots 630 maybe used for transmission of packet data, messages, and otherinformation. Each slot 630 may be assigned with or without subbandmultiplexing to one or more active terminals. Each slot 630 may also beused to send an overhead message to multiple terminals.

Various other frame structures may also be designed for use, and this iswithin the scope of the invention. For example, the uplink phase mayinclude a rate control segment used to send rate control information andan acknowledgment segment used to send acknowledgment data. As anotherexample, the frame may be partitioned into multiple uplink and downlinkphases, and different phases may be used for different types oftransmission such as traffic data, pilot, rate signaling, andacknowledgement.

Subband multiplexing can substantially reduce the amount of resourcesneeded to support the transmission of pilot and signaling on the uplink,as quantified below. However, various factors may need to be consideredin the implementation of subband multiplexing, such as (1) overheadsignaling for the assignment of subbands to the terminals, (2) timingoffset among the uplink transmissions received from the terminals, and(3) frequency offset among the uplink transmissions from the terminals.Each of these factors is described in further detail below.

Overhead signaling is needed to convey the subband assignment for eachterminal. For pilot and rate control information, each active terminalmay be assigned a specific subband group for each or for both types ofuplink transmission. This assignment may be made during call setup, andthe assigned subbands typically do not need to be repeated or changedfor every MAC frame.

If there are 24 subband groups for up to 24 terminals, then 5 bits wouldbe sufficient to identify the specific subband group assigned to aterminal. These 5 bits may be included in a control message sent to aterminal to put it into an active state. If the control message has alength of 80 bits, then the 5 bits for subband assignment would increasethe message length by approximately 6%.

The amount of overhead signaling would be greater if there isflexibility in forming the subband groups and/or if the groups may bedynamically assigned to the terminals. For example, if the number ofsubbands assigned for acknowledgment transmission can change from frameto frame, then higher amount of overhead signaling would be needed toconvey the subband assignment.

The multiple terminals allowed to transmit concurrently via subbandmultiplexing may be located throughout the system. If these terminalshave different distances to the access point, then the propagation timesfor the signals transmitted from these terminals would be different. Inthis case, if the terminals transmit their signals at the same time,then the access point would receive the signals from these terminals atdifferent times. The difference between the earliest and latest arrivingsignals at the access point would be dependent on the difference in theround trip delays for the terminals with respect to the access point.

The difference in arrival times for the signals from different terminalswould cut into the delay spread tolerance of the farther terminals. Asan example, for an access point with a coverage area of 50 meters inradius, the maximum difference in arrival times between the earliest andlatest arriving signals is approximately 330 nsec. This would representa significant portion of an 800 nsec cyclic prefix. Moreover, the effectof diminished delay spread tolerance is worst for the terminals at theedge of the coverage area, which are most in need of resilience tomultipath delay spread.

In an embodiment, to account for the difference in round trip delaysamong the active terminals, the uplink timing of each active terminal isadjusted so that its signal arrives within a particular time window atthe access point. A timing adjustment loop may be maintained for eachactive terminal and would estimate the round trip delay for theterminal. The uplink transmission from the terminal would then beadvanced or delayed by an amount determined by the estimated round tripdelay such that the uplink transmissions from all active terminalsarrive within the particular time window at the access point.

The timing adjustment for each active terminal may be derived based onthe pilot or some other uplink transmission from the terminal. Forexample, the uplink pilot may be correlated against a copy of the pilotby the access point. The result of the correlation is an indication ofwhether the received pilot is early or late with respect to the pilotsfrom the other terminals. A 1-bit timing adjustment value may then besent to the terminal to direct it to advance or retard its timing by aparticular amount (e.g., ±one sample period).

If subband multiplexing is used to permit simultaneous transmission bymultiple terminals on their assigned subbands, then the signals fromnearby terminals may cause substantial interference to the signals fromfaraway terminals if all terminals transmit at full power. Inparticular, it can be shown that frequency offset among the terminalscan result in inter-subband interference. This interference can causedegradation in the channel estimate derived from uplink pilots and/orincrease the bit error rate of uplink data transmissions. To mitigatethe effects of inter-subband interference, the terminals may be powercontrolled so that the nearby terminals do not cause excessiveinterference to faraway terminals.

The effect of interference from nearby terminals was investigated, andit was discovered that power control may be applied coarsely to mitigatethe inter-subband interference effect. In particular, it was found thatif the maximum frequency offset among the terminals is 300 Hz or less,then by limiting the SNRs of the nearby terminals to 40 dB or less,there would be a loss of 1 dB or less in the SNRs of the otherterminals. And if the frequency offset among the terminals is 1000 Hz orless, then the SNRs of the nearby terminals need to be limited to 27 dBto ensure 1 dB or less of loss in the SNRs of the other terminals. Ifthe SNR needed to achieve the highest rate supported by the OFDM systemis less than 27 dB, then limiting the SNR of nearby terminals to 27 dB(or 40 dB) would not have any impact on the maximum supported data ratesfor the nearby terminals.

The coarse power control requirements stated above may be achieved witha slow power control loop. For example, control messages may be sentwhen and as needed to adjust the uplink power of nearby terminals (e.g.,when the power level changes due to movement by these terminals). Eachterminal may be informed of the initial transmit power level to use forthe uplink when accessing the system as part of the call setup.

The groups of subbands may be assigned to the active terminals in amanner to mitigate the effect of inter-subband interference. Inparticular, terminals with high received SNRs may be assigned subbandsnear each other. Terminals with low received SNRs may be assignedsubbands near each other, but away from the subbands assigned to theterminals with high received SNRs.

The ability to have up to Q simultaneous uplink pilot transmissionsreduces the overhead for pilot by a factor of up to Q. The improvementcan be significant since the uplink pilot transmission can represent alarge portion of the uplink phase. The amount of improvement may bequantified for an exemplary OFDM system.

In this exemplary OFDM system, the system bandwidth is W=20 MHz andN=256. Each sample period has a duration of 50 nsec. A cyclic prefix of800 nsec (or Cp=16 samples) is used, and each OFDM symbol has a durationof 13.6 μsec (or N+Cp=272 samples). The uplink pilot is transmitted ineach MAC frame, which has a duration of 5 msec or 367 OFDM symbols. Thepilot transmission from each terminal needs to have total energy of 4symbol periods×full transmit power. If there are K active terminals,then the total number of symbol periods used for pilot transmissionswithout subband multiplexing is 4·K. For K=12, 48 symbol periods wouldbe used for uplink pilot transmission, which would representapproximately 13.1% of the 367 symbols in the MAC frame. The pilotoverhead would increase to 26.2% of the MAC frame if there are K=24active terminals.

If the K active terminals are assigned to K groups of subbands and areallowed to transmit the uplink pilot simultaneously, then only 4 symbolperiods would be required in each MAC frame for the uplink pilot. Theuse of subband multiplexing for the uplink pilot reduces the overhead to1.1% of the MAC frame for K=12 and 2.2% for K=24. This represents asignificant saving of 12% and 24% for K=12 and 24, respectively, in theamount of overhead required for uplink pilot transmission.

FIG. 8A shows a plot of the amount of saving in uplink pilottransmission for different number of active terminals for the exemplaryOFDM system described above. As shown in FIG. 8A, the amount of savingincreases approximately linearly with the number of terminals.

The amount of saving for an exemplary OFDM system that supports Q_(R)simultaneous uplink rate control transmissions may also be quantified.This exemplary OFDM system has M=224 usable subbands and uses BPSKmodulation with a rate 1/3 code. The number of information bits permodulation symbol is 1/3, and approximately 75 information bits may besent on the 224 usable subbands for each symbol period. If each terminalsends 15 bits or less of rate control information for each MAC frame,then approximately 5 terminals may be accommodated simultaneously on thesame OFDM symbol. Without subband multiplexing, 5 OFDM symbols wouldneed to be assigned to the 5 terminals for their rate controlinformation (where each OFDM symbol would contain a large amount ofpadding for the unused bits). With subband multiplexing, the same ratecontrol information may be sent within one OFDM symbol, which wouldrepresent an 80% saving.

The amount of savings with subband multiplexing is even greater for somediversity transmission modes. For a space-time transmit diversity (STTD)scheme, each pair of modulation symbols (denoted as s₁ and s₂) istransmitted over two symbol periods from two transmit antennas. Thefirst antenna transmits a vector x₁=[s₁ s₂*]^(T) over 2 symbol periodsand the second antenna transmits a vector x₂=[s₂ −s₁*]^(T) over the same2 symbol periods. The transmission unit for STTD is effectively two OFDMsymbols. With subband multiplexing, rate control information for 10terminals may be sent in 2 OFDM symbols, which is substantially lessthan the 20 OFDM symbols that would be needed if each terminal transmitsits rate control information on a separate pair of OFDM symbols.

The amount of saving is even greater for a diversity transmission modethat uses 4 antennas and has a transmission unit of 4 OFDM symbols. Forthis diversity transmission mode, 15 terminals may be subbandmultiplexed onto one 4-symbol period. The rate control information forthe 15 terminals may be sent in 4 OFDM symbols with subbandmultiplexing, which is substantially less than the 60 OFDM symbols thatwould be needed if each terminal transmits its rate control informationon a separate set of four OFDM symbols.

FIG. 8B shows a plot of the amount of saving in uplink rate controltransmission for different number of active terminals for an exemplaryOFDM system. For this system, up to 12 terminals may be multiplexedtogether using subband multiplexing. Each terminal may be assigned 18subbands, with each subband capable of carrying 3 information bits. The12 terminals may each be able to transmit 108 information bits in their18 assigned subbands in 2 symbol periods. This is much less than the 24symbol periods that would be needed by the 12 terminals without subbandmultiplexing. If 12 terminals are present, then a saving of 22 symbolsmay be achieved, which represents approximately 6% of the MAC frame with367 OFDM symbols. And if 24 terminals are present, then a saving of 44symbols may be realized, which represents approximately 12% of the MACframe. As shown in FIG. 8B, the amount of saving increases approximatelylinearly with the number of terminals.

FIG. 8C shows plots of the amount of saving resulting from subbandmultiplexing of the pilot, rate control, and acknowledgment on theuplink. In plot 812, the pilot and rate control information for multipleterminals are subband multiplexed in the pilot and rate controlsegments, respectively. The acknowledgment is not considered for thiscase. In plot 814, the pilot, rate control information, andacknowledgment for multiple terminals are subband multiplexed in thepilot, rate control, and acknowledgment segments, respectively.

As can be seen from the plots in FIG. 8C, the amount of saving increasesapproximately linearly with the number of terminals multiplexedtogether. Moreover, the amount of saving increases as more types ofinformation are multiplexed. It can be seen that subband multiplexingcan substantially reduce the amount of overhead for pilot and signaling,so that more of the available resources may be advantageously used fordata transmission.

FIG. 7 is a block diagram of an embodiment of an access point 110 x anda terminal 120 x, which are capable of supporting subband multiplexingfor the uplink. At access point 110 x, traffic data is provided from adata source 708 to a TX data processor 710, which formats, codes, andinterleaves the traffic data to provide coded data. The data rate andcoding may be determined by a rate control and a coding control,respectively, provided by a controller 730.

An OFDM modulator 720 receives and processes the coded data and pilotsymbols to provide a stream of OFDM symbols. The processing by OFDMmodulator 720 may include (1) modulating the coded data to formmodulation symbols, (2) multiplexing the modulation symbols with pilotsymbols, (3) transforming the modulation and pilot symbols to obtaintransformed symbols, and (4) appending a cyclic prefix to eachtransformed symbol to form a corresponding OFDM symbol.

A transmitter unit (TMTR) 722 then receives and converts the stream ofOFDM symbols into one or more analog signals and further conditions(e.g., amplifies, filters, and upconverts) the analog signals togenerate a downlink modulated signal suitable for transmission over thewireless channel. The modulated signal is then transmitted via anantenna 724 to the terminals.

At terminal 120 x, the downlink modulated signal is received by antenna752 and provided to a receiver unit (RCVR) 754. Receiver unit 754conditions (e.g., filters, amplifies, and downconverts) the receivedsignal and digitizes the conditioned signal to provide samples.

An OFDM demodulator 756 then removes the cyclic prefix appended to eachOFDM symbol, transforms each received transformed symbol using an FFT,and demodulates the received modulation symbols to provide demodulateddata. An RX data processor 758 then decodes the demodulated data torecover the transmitted traffic data, which is provided to a data sink760. The processing by OFDM demodulator 756 and RX data processor 758 iscomplementary to that performed by OFDM modulator 720 and TX dataprocessor 710, respectively, at access point 110 x.

As shown in FIG. 7, OFDM demodulator 756 may derive channel estimatesand provide these channel estimates to a controller 770. RX dataprocessor 758 may provide the status of each received packet. Based onthe various types of information received from OFDM demodulator 756 andRX data processor 758, controller 770 may determine or select aparticular rate for each transmission channel. Uplink pilot andsignaling information (e.g., the rates to use for downlink datatransmission, acknowledgments for received packets, and so on), may beprovided by controller 770, processed by a TX data processor 782,modulated by an OFDM modulator 784, conditioned by a transmitter unit786, and transmitted by antenna 752 back to access point 110 x. Theuplink pilot and signaling information may be sent on group(s) ofsubbands assigned to terminal 120 x for these types of transmissions.

At access point 110 x, the uplink modulated signal from terminal 120 xis received by antenna 724, conditioned by a receiver unit 742,demodulated by an OFDM demodulator 744, and processed by a RX dataprocessor 746 to recover the pilot and signaling information transmittedby the terminal. The recovered signaling information is provided tocontroller 730 and used to control the processing of the downlink datatransmission to the terminal. For example, the rate on each transmissionchannel may be determined based on the rate control information providedby the terminal, or may be determined based on the channel estimatesfrom the terminal. The received acknowledgment may be used to initiateretransmission of packets received in error by the terminal. Controller730 may also derive the enhanced channel frequency response for eachterminal based on the uplink pilot transmitted on the assigned subbands,as described above.

Controllers 730 and 770 direct the operation at the access point andterminal, respectively. Memories 732 and 772 provide storage for programcodes and data used by controllers 730 and 770, respectively.

The uplink pilot and signaling transmission techniques described hereinmay be implemented by various means. For example, these techniques maybe implemented in hardware, software, or a combination thereof. For ahardware implementation, the elements used to implement any one or acombination of the techniques may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof.

For a software implementation, these techniques may be implemented withmodules (e.g., procedures, functions, and so on) that perform thefunctions described herein. The software codes may be stored in a memoryunit (e.g., memory units 732 or 772 in FIG. 7) and executed by aprocessor (e.g., controller 730 or 770). The memory unit may beimplemented within the processor or external to the processor, in whichcase it can be communicatively coupled to the processor via variousmeans as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for assigning pilot signals for transmission, comprising:assigning a first group of subbands to pilot signals of a firstterminal, wherein each subband of the first group is non-contiguous withany other subband of the first group; and assigning a second group ofsubbands to pilot signals a second terminal, wherein each subband of thesecond group is non-contiguous with any other subband of the secondgroup.
 2. The method of claim 1, wherein each of the first group and thesecond group includes a same number of subbands.
 3. The method of claim1, wherein each of the first group and the second group each includes adifferent number of subbands.
 4. The method of claim 1, wherein at leastone subband in the first group is adjacent to at least one subband inthe second group.
 5. The method of claim 1, wherein each subband of thefirst group is spaced N subbands from a nearest subband of the firstgroup.
 6. The method of claim 5, wherein each subband of the secondgroup is spaced M subbands from a nearest subband of the second group.7. The method of claim 6, wherein N and M are equal.
 8. The method ofclaim 6, wherein N and M are not equal.
 9. The method of claim 1,wherein each subband of the first group is uniformly distributed acrossa plurality of usable subbands.
 10. The method of claim 1, wherein eachsubband of the first group is non-uniformly distributed across aplurality of usable subbands.
 11. The method of claim 1, wherein thepilot signals are for forward link transmission.
 12. An apparatuscomprising: a memory; and a processor configured to assign a first groupof subbands to pilot signals of a first terminal and assign a secondgroup of subbands to pilot signals a second terminal, wherein eachsubband of the first group is non-contiguous with any other frequencyband of the first group.
 13. The apparatus of claim 12, wherein each ofthe first group and the second group includes a same number of subbands.14. The apparatus of claim 12, wherein each of the first group and thesecond group each includes a different number of subbands.
 15. Theapparatus of claim 12, wherein at least one subband in the first groupis adjacent to at least one subband in the second group.
 16. Theapparatus of claim 12, wherein each subband of the first group is spacedN subbands from a nearest subband of the first group.
 17. The apparatusof claim 16, wherein each subband of the second group is spaced Msubbands from a nearest subband of the second group.
 18. The apparatusof claim 17, wherein N and M are equal.
 19. The apparatus of claim 17,wherein N and M are not equal.
 20. The apparatus of claim 13, whereineach subband of the first group is uniformly distributed across aplurality of usable subbands.
 21. An apparatus comprising: means forassigning a first group of non-contiguous subbands to pilot signals of afirst terminal; and means for assigning a second group of non-contiguoussubbands to pilot signals of a second terminal.
 22. The apparatus ofclaim 21, wherein each of the first group and the second group includesa same number of subbands.
 23. The apparatus of claim 21, wherein eachof the first group and the second group each includes a different numberof subbands.
 24. The apparatus of claim 21, wherein each subband of thefirst group is spaced N subbands from a nearest subband of the firstgroup.
 25. The apparatus of claim 24, wherein each subband of the secondgroup is spaced M subbands from a nearest subband of the second group.26. The apparatus of claim 25, wherein N and M are equal.
 27. Theapparatus of claim 25, wherein N and M are not equal.
 28. The apparatusof claim 21, wherein each subband of the first group is uniformlydistributed across a plurality of usable subbands.
 29. The apparatus ofclaim 21, wherein each subband of the first group is non-uniformlydistributed across a plurality of usable subbands.
 30. The apparatus ofclaim 21, wherein the pilot signals are for forward link transmission.